On Wed, Jul 18, 2018 at 1:52 AM, Paul Burton <paul.burton@xxxxxxxx> wrote: > Hi Huacai, > > On Fri, Jul 13, 2018 at 03:37:57PM +0800, Huacai Chen wrote: >> Linux expects that if a CPU modifies a memory location, then that >> modification will eventually become visible to other CPUs in the system. >> >> On Loongson-3 processor with SFB (Store Fill Buffer), loads may be >> prioritised over stores so it is possible for a store operation to be >> postponed if a polling loop immediately follows it. If the variable >> being polled indirectly depends on the outstanding store [for example, >> another CPU may be polling the variable that is pending modification] >> then there is the potential for deadlock if interrupts are disabled. >> This deadlock occurs in qspinlock code. >> >> This patch changes the definition of cpu_relax() to smp_mb() for >> Loongson-3, forcing a flushing of the SFB on SMP systems before the >> next load takes place. If the Kernel is not compiled for SMP support, >> this will expand to a barrier() as before. >> >> References: 534be1d5a2da940 (ARM: 6194/1: change definition of cpu_relax() for ARM11MPCore) >> Cc: stable@xxxxxxxxxxxxxxx >> Signed-off-by: Huacai Chen <chenhc@xxxxxxxxxx> >> --- >> arch/mips/include/asm/processor.h | 10 ++++++++++ >> 1 file changed, 10 insertions(+) >> >> diff --git a/arch/mips/include/asm/processor.h b/arch/mips/include/asm/processor.h >> index af34afb..a8c4a3a 100644 >> --- a/arch/mips/include/asm/processor.h >> +++ b/arch/mips/include/asm/processor.h >> @@ -386,7 +386,17 @@ unsigned long get_wchan(struct task_struct *p); >> #define KSTK_ESP(tsk) (task_pt_regs(tsk)->regs[29]) >> #define KSTK_STATUS(tsk) (task_pt_regs(tsk)->cp0_status) >> >> +#ifdef CONFIG_CPU_LOONGSON3 >> +/* >> + * Loongson-3's SFB (Store-Fill-Buffer) may get starved when stuck in a read >> + * loop. Since spin loops of any kind should have a cpu_relax() in them, force >> + * a Store-Fill-Buffer flush from cpu_relax() such that any pending writes will >> + * become available as expected. >> + */ > > I think "may starve writes" or "may queue writes indefinitely" would be > clearer than "may get starved". Need I change the comment and resend? Or you change the comment and get merged? Huacai > >> +#define cpu_relax() smp_mb() >> +#else >> #define cpu_relax() barrier() >> +#endif >> >> /* >> * Return_address is a replacement for __builtin_return_address(count) >> -- >> 2.7.0 > > Apart from the comment above though this looks better to me. > > Re-copying the LKMM maintainers - are you happy(ish) with this? > > Thanks, > Paul
